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NASA NTRS2004

The Burst and Transient Source Experiment (BATSE) Earth Occultation Catalog of Low-Energy Gamma-Ray Sources

Harmon, B. A.↗

NASA NTRS2003

The Burst and Transient Source Experiment(BATSE)Earth Occultation Catalog of Low-Energy Gamma-Ray Sources. Short title: BATSE Earth Occultation Catalog v2.0

The Burst and Transient Source Experiment (BATSE), aboard the Compton Gamma Ray Observatory (CGRO), provided a record of the low-energy gamma-ray sky (approx. 20-1000 keV) between 1991 April and 2000 May (9.1y). BATSE monitored the high energy sky using the Earth occultation technique (EOT) for point sources whose emission extended for times on the order of the CGRO orbital period (approx. 92m) or greater. Using the EOT to extract flux - 2 - information, a catalog of sources using data from the BATSE large area detectors has been prepared. The first part of the catalog consists of results from the all-sky monitoring of 58 sources, mostly Galactic, with intrinsic variability on timescales of hours to years. For these sources, we have included tables of flux and spectral data, and outburst times for transients. Light curves (or flux histories) covering the entire nine mission are being placed on the world wide web. We then performed a deep-sampling of these 58 objects, plus a selection of 121 more objects, combining data from the entire 9.ly BATSE dataset. Source types considered were primarily accreting binaries, but a small number of representative active galaxies, X-ray-emitting stars, and supernova remnants were also included. The sample represents a compilation of sources monitored and/or discovered with BATSE and other high energy instruments between 1991 and 2000, known sources taken from the HEAO 1 A-4 (Levine et al. 1984) and Macomb and Gehrels (1999) catalogs. The deep sample results include definite detections of 82 objects and possible detections of 36 additional objects. The definite detections spanned three classes of sources: accreting black hole and neutron star binaries, active galaxies and supernova remnants. The average fluxes measured for the fourth class, the X-ray emitting stars, were below the confidence limit for definite detection. Flux data for the deep sample are presented in four energy bands: 20-40, 40-70, 70-160, and 160-430 keV. The limiting average flux level (9.1 y) for the sample varies from 3.5 to 20 mCrab (5delta) between 20 and 430 keV, depending on systematic error, which in turn is primarily dependent on the sky location. To strengthen the credibility of detection of weaker sources (approx.5-25 mCrab), we generated Earth occultation images, searched for periodic behavior using FFT and epoch folding methods, and critically evaluated the energy-dependent emission in the four flux bands. The deep sample results are intended for guidance in performing future all-sky surveys or pointed observations in the hard X-ray and low-energy gamma-ray band, as well as more detailed studies with the BATSE EOT.

Harmon, B. A.↗

NASA NTRS2008

Understanding Black Hole X-ray Binaries: The Case of Cygnus X-1

Black Hole X-ray Binaries are known to display distinct emission states that differ in their X-ray spectra, their X-ray timing properties (on times scales less than 1 s) and their radio emission. In recent years monitoring observations, specially with NASA's Rossi X-ray Timing Explorer (RXTE), have provided us with detailed empirical modeling of the phenomenology of the different states as well as a unification scheme of the long term evolution of black holes, transient and persistent, in terms of these states. Observations of the persistent High Mass X-ray Binary (HMXB) Cygnus X-l have been at the forefront of learning about black hole states since its optical identification through a state transition in 1973. In this talk I will present in depth studies of several different aspects of the accretion process in this system. The main data base for these studies is an ongoing RXTE and Ryle radio telescope bi-weekly monitoring campaign that started in 1997. I will discuss high-resolution timing results, especially power spectra, which first gave rise to the Lorentzian description now widely used for black hole and neutron star binaries, and time lags, which we found to be especially well suited to identify state transitions. The evolution of spectral, timing, and radio parameters over years will be shown, including the rms-flux relation and the observation of a clearly correlated radio/x-ray flare. We also observed Cygnus X-1 with INTEGRAL, which allowed us to extend timing and spectral studies to higher energies, with XMM, which provided strong constraints on the parameters of the 6.4 keV iron fluorescence line, and with Chandra, which provided the most in depth study to date of the stellar wind in this system. Models based on the physical conditions in the accretion region are still mainly concentrated on the one or other of the observational areas but they are expanding: as an example I will review results from a jet model for the quantitative description of the radio through X-ray spectra. I will conclude with an outlook on a truly multi-instrument observing campaign of Cygnus X-1 that was performed in 2008 April in order to better constrain the jet models mentioned above (and provide a unique data set for cross-calibration).

Pottschmidt, Katja↗

NASA NTRS2012

Anomalous Low States and Long Term Variability in the Black Hole Binary LMC X-3

Rossi X-my Timing Explorer observations of the black hole binary LMC X-3 reveal an extended very low X-ray state lasting from 2003 December 13 until 2004 March 18, unprecedented both in terms of its low luminosity (>15 times fainter than ever before seen in this source) and long duration (approx 3 times longer than a typical low/hard state excursion). During this event little to no source variability is observed on timescales of approx hours-weeks, and the X-ray spectrum implies an upper limit of 1.2 x 10(exp 35) erg/s, Five years later another extended low state occurs, lasting from 2008 December 11 until 2009 June 17. This event lasts nearly twice as long as the first, and while significant variability is observed, the source remains reliably in the low/hard spectral state for the approx 188 day duration. These episodes share some characteristics with the "anomalous low states" in the neutron star binary Her X-I. The average period and amplitude of the Variability of LMC X-3 have different values between these episodes. We characterize the long-term variability of LMC X-3 before and after the two events using conventional and nonlinear time series analysis methods, and show that, as is the case in Her X-I, the characteristic amplitude of the variability is related to its characteristic timescale. Furthermore, the relation is in the same direction in both systems. This suggests that a similar mechanism gives rise to the long-term variability, which in the case of Her X-I is reliably modeled with a tilted, warped precessing accretion disk.

Smale, Alan P.↗

NASA NTRS1979

A search for X-ray pulsations from the Galactic Centre

A search has been conducted for periodic X-ray behavior in the Galactic Center, a region known to contain several point sources. No pulsations are detected in the range from about 2 sec to 2000 sec. The assumption that part of the steady flux from the Galactic Center is due to the steady contribution of bursters is discussed. It is also noted that the absence of pulsations does not necessarily undermine the proposed binary-neutron star models for the burst sources.

Cordova, F. A.↗

NASA NTRS1982

X-ray properties of the Be/X-ray systems 2S0114+650 = LSI+65 deg 010

Results are presented from experiments on the Einstein Observatory, HEAO-1 and OSO-8 on the temporal and spectral properties of 2S0114+650. In a 12 hour Einstein MPC and SSS observation two episodes of flaring occurred by an order of magnitude over about 1 hour. Variability on shorter time scales showed a preferred period of 14.9 minutes, but periodic pulsations were not seen in the HEAO-1 and OSO-8 data. There is some evidence that the spectrum is steeper when the source is quiescent at low luminosity, but no large spectral changes attended the flares. Absorption column densities were consistent with interstellar reddening of the proposed companion, the Be star LSI+65 deg 010. In the minute and hour variability and in the spectral character, 2S0114+650 is similar to other Be star-neutron star binary X-ray sources. Variations over several days in the OSO-8 data suggest orbital effects.

Koenigsberger, G.↗

NASA NTRS1983

X-ray properties of the Be/X-ray system 28 0114+650 = LSI +65deg 010

Results are presented from experiments on the Einstein Observatory, HEAO 1, and OSO 8 on the temporal and spectral properties of 2S 0114+650. In a 12 hr Einstein monitor proportional counter and solid state spectrometer observation, two episodes of flaring occurred by an order of magnitude over about 1 hr. Variability on shorter time scales showed a preferred period of 14.9 minutes, but periodic pulsations were not seen in the HEAO 1 and OSO 8 data. The spectra above 1 keV were typical of accreting X-ray pulsars; the power law was E exp -alpha with alpha of about 1.2 for the number spectrum up to a cutoff at 14 keV. There is some evidence that the spectrum is steeper when the source is quiescent at low luminosity, but no large spectral changes attended the flares. Absorption column densities were consistent with interstellar reddening of the proposed companion, the Be star LSI +65 deg010. In the minute and hour variability and in the spectral character, 2S0114+650 is similar to other Be star-neutron star binary X-ray sources. Variations over several days in the OSO 8 data suggest orbital effects.

Koenigsberger, G.↗

NASA NTRS1985

Observation of gamma-rays from LMC X-4 above 10(16) eV

Data from the University of Adelaide air shower array at Buckland Park taken over a three year period have been analyzed to search for evidence of ultra high energy gamma-ray emission from neutron star binary X-ray sources having known orbital periods. The detection of UHE gamma-rays from LMC X-4 above 10 to the 16th power eV, is reported; the first extragalactic object to be positively detected at these energies.

Protheroe, R. J.↗

NASA NTRS1986

Simultaneous X-ray and optical observations of GX 339-4 in an X-ray high state

Optical emission, soft X-rays, and hard X-rays have been observed simultaneously from the black hole candidate GX 3339-4 in a typical high state. Each of these components is interpreted as arising from a characteristic region of the accretion disk. Considered as a black hole, GX 339-4 lacks the variable 2 keV blackbody component that would be emitted from the 'solid' surface of a neutron star in the low-mass neutron star binaries. The X-ray emission in the high state is therefore extremely soft and stable in time. The intermediate disk region is optically thick and geometrically thin. The observed soft X-ray component with a 'disk blackbody' spectrum comes from this region. The innermost disk region is dynamically unstable due to the effect of general relativity and possibly due to radiation pressure. This region is responsible for the generation of the hard X-ray tail through Comptonization.

Makishima, K.↗

NASA NTRS1988

An all sky study of fast X-ray transients

In the HEAO 1 A-2 survey of fast X-ray transients, a search of 2 to 20 keV scanning data was made for brief increases in flux, greater than 4 millicrabs, on timescales approximately 1 to 10 to the 4th s above a 12-hour average. The search was divided into two regions, and all-sky survey of the Galaxy, and a survey of the Magellanic Clouds. In the latter, 37 events were found and identified with 4 of the 5 brighest sources in the LMC, plus 2 as flares from a foreground star. Np X-ray bursts, no gamma-ray bursts, and no events from the SMC were found. In the all-sky survey, after excluding well-known variable sources, out of the equivalent of approximately 104 days of data, 15 events were found which falling into 2 broad categories: flares from coronal sources, roughly isotropically distributed, with optically thin thermal spectra; and harder fast transients apparently distributed near the Galactic plane. The first were identified as flares from ubiquitous active cool dwarf stars. It was hypothesized that the second may have been from distant Be-neutron star binaries. However, at least two of the harder, more luminous events remained unidentified. Future research should examine the nature of these rare events, and how they may fit into a hierarchy of hard fast transients from gamma-ray bursts to outbursts from pulsar systems.

Connors, Alanna↗

NASA NTRS1992

Formation of very strongly magnetized neutron stars - Implications for gamma-ray bursts

It is proposed that the main observational signature of magnetars, high-field neutron stars, is gamma-ray bursts powered by their vast reservoirs of magnetic energy. If they acquire large recoils, most magnetars are unbound from the Galaxy or reside in an extended, weakly bound Galactic corona. There is evidence that the soft gamma repeaters are young magnetars. It is argued that a convective dynamo can also generate a very strong dipole field after the merger of a neutron star binary, but only if the merged star survives for as long as about 10-100 ms. Several mechanisms which could impart a large recoil to these stars at birth, sufficient to escape from the Galactic disk, are discussed.

Duncan, Robert C.↗

NASA NTRS1993

Relativistically expanding pair plasmas as bursting sources of cosmic gamma rays

High temperature shock heating of relativistically expanding plasmas produced in neutron star binary mergers provides a model for cosmic gamma ray burst sources. This requires the fireball resulting from the merger to have a very high entropy per baryon, mechanisms for which are discussed. The energy, temporal structure and spectrum produced by the blast wave and reverse shock as the fireball is decelerated in an external medium are comparable to those observed, as is the frequency of occurrence and the characteristics of the spatial distribution of the events. Difficulties common to all cosmological gamma ray burst scenarios concern the total amount of energy ultimately appearing at gamma ray energies, the time scales, the spectrum, and the great variety of time profiles. A very general mechanism which overcomes these problems is presented. Situations where the pair plasma is created in regions which are relatively free of baryons are discussed. The effect of the interaction of the expanding fireball with the external medium is considered.

Meszaros, Peter↗

NASA NTRS1995

Gamma Ray Astronomy; E2 Symposium of COSPAR Scientific Commission E, COSPAR Scientific Assembly, 30th, Hamburg, Germany, July 11-21, 1994

This issue on gamma ray astronomy presents many results from observations made with the Compton Gamma Ray Observatory (CGRO) and other spaceborne experiments and includes studies of Seyfert galaxies, X-ray binaries, neutron stars, diffuse cosmic background radiation, pulsars, blazars, gamma ray bursts, the galactic center, and the distribution of Aluminum-26 in the galaxy. The issue begins with a survey of Compton Telescope (COMPTEL) results and concludes with future prospects for the field.

Gehrels, N.↗

NASA NTRS1999

Preview of the BATSE Earth Occultation Catalog of Low Energy Gamma Ray Sources

The Burst and Transient Source Experiment (BATSE) aboard the Compton Gamma Ray Observatory (CGRO) has been detecting and monitoring point sources in the high energy sky since 1991. Although BATSE is best known for gamma ray bursts, it also monitors the sky for longer-lived sources of radiation. Using the Earth occultation technique to extract flux information, a catalog is being prepared of about 150 sources potential emission in the large area detectors (20-1000 keV). The catalog will contain light curves, representative spectra, and parametric data for black hole and neutron star binaries, active galaxies, and super-nova remnants. In this preview, we present light curves for persistent and transient sources, and also show examples of what type of information can be obtained from the BATSE Earth occultation database. Options for making the data easily accessible as an "on line" WWW document are being explored.

Harmon, B. A.↗

NASA NTRS2001

Simultaneous Spectral and Timing Observations of Accreting Neuron Stars

The goal of this proposal was to perform simultaneous x-ray spectral and millisecond timing observations of accreting neutron stars to further our understanding of their accretion dynamics and in the hope of using these systems as probes of the physics of strong gravitational fields. Observations of the neutron star binaries 4U0614+091, 4U1728-34, 4U1820-30, and Cyg X-2 were carried out with RXTE and BeppoSAX, ASCA, and Chandra (not all simultaneously). In addition, archival data were analyzed for 4U0614+091 and 4U1820-30. This investigation led to publication of three papers in peer-reviewed journals. These are listed below. In addition, the results were presented at several meetings including the two poster presentations listed below. Dr. Santina Piraino visited SAO for 4 months during 2000 to collaborate on analysis of the data from NAG5-8408 and NAG5-9104.

Kaaret, P.↗

NASA NTRS2011

Data Analysis And Polarization Measurements With GEMS

The Gravity and Extreme Magnetism SMEX (GEMS) mission was selected by NASA for flight in 2014. GEMS will make the first sensitive survey of X-ray polarization across a wide range of source classes including black hole and neutron star binaries, AGN of different types, rotation and accretion-powered pulsars, magnetars, shell supernova remnants and pulsar wind nebulae. GEMS employs grazing-incidence foil mirrors and novel time-projection chamber (TPC) polarimeters leveraging the photoelectric effect. The GEMS detectors image the charge tracks of photoelectrons produced by 2 - 10 keV X-rays. The initial direction of the photoelectron is determined by the linear polarization of the photon. We present an overview of the data analysis challenges and methods for GEMS, including procedures for producing optimally filtered images of the charge tracks and estimating their initial directions. We illustrate our methods using laboratory measurements of polarized and unpolarized X-rays with flight-like detectors as well as from simulated tracks. We also present detailed simulations exploring the statistics of polarization measurements appropriate for GEMS, and make comparisons with previous work.

Stohmayer, Tod↗

NASA NTRS2012

Implications for the Origin of GRB 051103 from LIGO Observations

We present the results of a LIGO search for gravitational waves (GWs) associated with GRB 051103, a short-duration hard-spectrum gamma-ray burst whose electromagnetically determined sky position is coincident with the spiral galaxy M81, which is 3.6Mpc from Earth. Possible progenitors for short-hard GRBs include compact object mergers and soft gamma repeater (SGR) giant flares. A merger progenitor would produce a characteristic GW signal that should be detectable at the distance of M81, while GW emission from an SGR is not expected to be detectable at that distance. We found no evidence of a GW signal associated with GRB 051103. Assuming weakly beamed gamma-ray emission with a jet semi-angle of 30. we exclude a binary neutron star merger in M81 as the progenitor with a confidence of 98%. Neutron star-black hole mergers are excluded with > 99% confidence. If the event occurred in M81 our findings support the hypothesis that GRB 051103 was due to an SGR giant flare, making it the most distant extragalactic magnetar observed to date.

Bizouard, M. A.↗

NASA NTRS2012

Seeking Counterparts to Advanced LIGO/Virgo Transients with Swift

Binary neutron star (NS) mergers are among the most promising astrophysical sources of gravitational wave emission for Advanced LIGO and Advanced Virgo, expected to be operational in 2015 . Finding electromagnetic counterparts to these signals will be essential to placing them in an astronomical context. The Swift satellite carries a sensitive X-ray telescope (XRT), and can respond to target-of-opportunity requests within 1-2 hours, and so is uniquely poised to find the X-ray counterparts to LIGO / Virgo triggers. Assuming NS mergers are the progenitors of short gamma-ray bursts (GRBs), some percentage of LIGO/Virgo triggers will be accompanied by X-ray band afterglows that are brighter than 10(exp -12) ergs/s/sq cm in the XRT band one day after the trigger time. We find that a soft X-ray transient of this flux is bright enough to be extremely rare, and so could be confidently associated with even a moderately localized GW signal. We examine two possible search strategies with the Swift XRT to find bright transients in LIGO/Virgo error boxes. In the first strategy, XRT could search a volume of space with a approx.100 Mpc radius by observing approx 30 galaxies over the course of a day, with sufficient depth to observe the expected X-ray afterglow. For an extended LIGO / Virgo horizon distance, the XRT could employ very short 100 s exposures to cover an area of approx 35 square degrees in about a day, and still be sensitive enough to image GW discovered GRB afterglows. These strategies demonstrate that the high X-ray luminosity of short GRBs and the relatively low X-ray transient background combine to make high confidence discoveries of X-ray band counterparts to GW triggers possible, though challenging, with current satellite facilities.

Kanner, Jonah↗